Semiconductor devices, such as diodes and transistors are essential components for electronic devices. A continuing demand exists for new, alternative, less expensive and/or improved manufacturing processes for their production. Currently, a particular interest exists in processes for the production of flexible electronics components for use e.g. in RFID tags, flexible LED and LCD displays and photovoltaics. A very promising technique for producing flexible electronics is the so-called roll-to-roll (R2R) fabrication technique (also known as web processing or reel-to-reel processing) wherein thin-films are deposited on a flexible (plastic) substrate and processed into electrical components in a continuous way.
In an R2R process printing techniques (e.g. imprint, inkjet, or screen printing) and coating techniques (e.g. roll, slit coating or spray coating) are used in order to achieve high-throughput, low-cost manufacture of semiconducting devices, including photovoltaic cells and TFT circuitry for displays. Such techniques include the use of inks, i.e. liquid semiconductor, metal and dielectric precursors, which can be deposited on the substrate using a simple coating or printing technique. This way, flexible electronics may be fabricated at a fraction of the cost of traditional semiconductor manufacturing methods.
In order to realize flexible electronics for high-performance applications, such as UHF RFIDs and flexible displays, low-cost and high-throughput formation of high-mobility thin-film semiconductor layers on a flexible substrate is required. Further, the manufacturing process should support formation of structures having small feature size and high alignment accuracy. Commercially interesting candidates for a flexible plastic substrate material include polyethylene naphthalate (PEN) and polyethylene terephthalate (PET). These materials are low-cost materials with a high optical transparency and chemically compatible with most semiconductor processes. The maximum processing temperatures of these materials are however relatively low approximately (approx. 200° C. for PEN and 120° C. for PET).
Several liquid-based techniques for forming a semiconducting coating on a substrate are known. Organic semiconductor materials may be used in a low-temperature deposition technique in order to realize “plastic” TFT circuitry for LCD applications or “plastic” photovoltaic cells. However, the electron mobility and reliability of these organic semiconductors are still inferior to their amorphous silicon counterparts (approx. 1 cm2/Vs) so that integration of peripheral driver and control circuits is difficult to achieve. Alternatively, amorphous metal-oxide semiconductors like In—Ga—Zn—O (a-IGZO) may be formed on a plastic substrate using a low-temperature solution-based process. Although, the electron mobility of an a-IGZO layer is higher than a-Si, it is still limited to 20 cm2/Vs. Furthermore, the hole mobility is very low so that p-type metal-oxide semiconductor TFTs cannot be made. The inability to realize circuitry in a CMOS configuration poses a serious limitation on the use of this material in commercial applications. Hence, in summary, plastic and a-IGZO semiconducting materials are still substantially inferior to poly-crystalline silicon that offers highly stable electrical properties and sufficiently high mobility (>100 cm2/Vs) for electronics applications.
Techniques for liquid-based formation of silicon are known. For example, U.S. Pat. No. 6,541,354 and US2003/0229190 describe processes for forming silicon films using a solution containing a cyclic silane compound such as cyclopentasilane (CPS) and a solvent. Typically, the solution is spin-coated onto a substrate and subjected to a drying step in order to remove the solvent. Thereafter, a combined UV treatment and annealing step of the coated substrate at a temperature of around 300° C. is used to transform the coating layer in 30 minutes into an amorphous silicon layer. A further annealing step at 800° C. or exposure of the amorphous silicon layer to laser light may covert the amorphous layer into a poly-crystalline layer.
Further development of this process has been reported in various documents. For example, Zhang et. al. reported in their article “single-grain Si TFTs on flexible polyimide substrate from doctor-blade coated cyclopentasilane” the fabrication of TFTs on polyimide substrate by exposing a CPS coated substrate comprising small holes (“grain filters”) for one hour in a nitrogen atmosphere to at a temperature of 350° C. in order to transform the coating into amorphous Silicon. A multi-shot laser exposure was used in order to transform the amorphous silicon into a crystalline silicon layer wherein dehydrogenation of the layer was achieved by gradually increasing the energy density from 50 to 350 mJ/cm2 while decreasing the number of the shots for each energy density from 100 to 1.
The prior art thus describes the formation of poly-silicon films on a substrate on the basis of liquid silane compound based processes that include coating the substrate with liquid silane compound and transforming the coating into amorphous silicon by exposing the substrate with the coating to an annealing step at temperatures around 300° C. to 350° C. for a relatively long period of time (e.g. 10-30 minutes). In some cases before annealing the coating may be exposed to UV radiation in order to transform e.g. cyclic silane compounds such as CPS into polysilane. A subsequent laser irradiation step or a high-temperature pyrolysis step is used for transforming the amorphous silicon into polycrystalline silicon. Although high quality polycrystalline films were achieved, the processing temperature not sufficiently low enough for plastic substrate materials such as PET and PEN. In addition, the dehydrogenation step required many number of shots, possibly causing low throughput in the production.
Hence, there is a need for in the art for fast and efficient low-temperature formation of silicon using a liquid silicon precursor. In particular, there is a need in the art for efficient low temperature formation of amorphous, microcrystalline and poly-crystalline layers on plastic substrates using a liquid-based process.